This invention relates to an improved color television picture tube and particularly to a tube having a slit shadow mask and a line screen wherein portions of the screen are covered with a light absorbing material.
Color television picture tubes with screens including light absorbing materials incorporated as a black matrix have been in commercial use for several years. When the matrix screen concept was applied to tubes having slit shadow masks and line screens, the light absorbing areas were disposed as vertical lines between the phosphor lines. One such configuration is shown in U.S. Pat. No. 4,070,596 issued to Tsuneta et al. on Jan. 24, 1978.
There are two types of tubes in general use that have slit shadow masks and line screens. The first has continuous slits extending from the top to the bottom of the mask. In the second type, the slits are shorter apertures aligned in substantially parallel columns. The slits in each column are separated from each other by crossties, i.e., web portions, of the mask. It is this second type of tube which is improved by the present invention.
Three principal methods are used in fabricating screens of most tubes having slit masks with webs. Two of these methods produce continuous phosphor lines across the screen while the third produces phosphor lines, each of which comprises a series of line segments. In one method, a linear light source, aligned with the aperture columns, is used to photoexpose the screen. The length of the linear light source is such that the web pattern is not reproduced on the screen. (See U.S. Pat. No. 4,049,451 issued to H. B. Law, on Sept. 20, 1977.) In the second method, the linear light source is simulated by using a moving point light source. Each of these two methods produce a line screen wherein each line is separated by parallel light absorbing lines. However, in the third method, a stationary point light source is utilized to form the screen, thus producing line segments. In such a method, in addition to the light absorbing areas being disposed as vertical elements between the phosphor lines, the light absorbing areas are connected by additional light absorbing areas behind the crossties which were formed by the shadowing effect of the crossties. Such configuration is shown in U.S. Pat. No. 4,066,924 issued to W. Rublack on Jan. 3, 1978.
In all of the embodiments described in the foregoing mentioned patents, wherein the screens include a light absorbing matrix, the basic mask-screen structure is of a type referred to as negative tolerance in the X--X or horizontal direction. Negative tolerance occurs when the shadow mask apertures define an electron beam spot during tube operation which is larger or wider than a hole in the light absorbing matrix that contains a light emitting phosphor element. In a positive tolerance type matrix structure, the mask aperture defines an electron beam spot during tube operation which is smaller or narrower than the hole in the light absorbing matrix. A tube construction having a matrix line screen with positive tolerance in the X--X or horizontal direction is disclosed in U.S. Pat. No. 3,979,630 issued to D. D. VanOrmer on Sept. 7, 1976.
Each of the foregoing tube types has its peculiar disadvantages. For example, in the prior art matrix tubes, some light output must be sacrificed in order to obtain the desired enhanced contrast. In negative tolerance tubes, the light absorbing matrix covers about 40 percent of the screen area, thus providing a matrix transmission of about 60 percent. Matrix transmission is considerably increased in the positive tolerance matrix tube disclosed in U.S. Pat. No. 3,979,630. The highest matrix transmission noted for a positive tolerance matrix line tube in this patent is 88.6 percent. However, because of the positive tolerance aspect, the mask transmission is considerably less than the mask transmission in a negative tolerance tube thereby resulting in little difference in light output. In both types of prior art tubes, utilization of a light absorbing matrix permits an increase in glass transmission of the tube faceplate which allows substantial recovery of light output lost because of the matrix.